CN115655133B - Ground stress measuring method based on optical fiber strain sensing tubular column - Google Patents
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Abstract
The invention belongs to the technical field of ground stress measurement, and particularly relates to an optical fiber strain sensing pipe column and a ground stress measurement method, wherein the optical fiber strain sensing pipe column comprises a pipe column body and an optical fiber sensor, the pipe column body is provided with an inlet end for connecting a liquid injection pressurizing system, and strain can be generated by the pipe column body according to internal and external pressure changes; the optical fiber sensor is adhered to the peripheral wall of the pipe column body and can measure the strain of the pipe column body, and a plurality of sensing points which are arranged at intervals and positioning marks for marking the space positions of the sensing points are arranged on the optical fiber sensor. According to the invention, the winding type optical fiber strain sensing pipe column is adopted, the optical fiber sensor with high spatial resolution is installed in the stratum, different ground stress states of the positions of different sensing sections can be calculated according to the strain measured by different sensing points on the optical fiber strain sensing pipe column, and the real-time measurement of multiple layers of ground stress is realized. Compared with the traditional ground stress measuring method, the method is simpler to operate, the ground stress state of the continuous time-space domain can be measured, and the ground stress measuring capability is greatly improved.
Description
Technical Field
The invention belongs to the technical field of ground stress measurement, and particularly relates to a ground stress measurement method based on an optical fiber strain sensing tubular column.
Background
Ground stress refers to the in situ stress state of the formation. In geotechnical engineering fields such as drilling engineering, mining engineering and road engineering, ground stress is one of the key engineering parameters, and has received a great deal of attention. Therefore, how to accurately measure and predict the ground stress is always a research hotspot in the geotechnical engineering field.
However, the traditional ground stress measurement can only carry out static measurement of a single point position, the measurement result can only represent the ground stress state at a certain moment and a certain position, and the ground stress measurement can not be carried out again after the well cementation or production of the oil and gas well is completed; the higher the spatial resolution of the ground stress is, the more the point positions need to be measured, and the ground stress state at a certain point position or a certain depth can be measured in the traditional mode; in addition, the traditional ground stress measuring method needs to carry out a series of complex operations such as coring, drilling, sensor descending, hydraulic fracturing, flat jack embedding and the like, and the measuring difficulty is high.
Disclosure of Invention
The invention mainly aims to provide a ground stress measuring method based on an optical fiber strain sensing tubular column, and aims to solve the technical problems that the ground stress measuring method in the prior art can only carry out single-point static measurement and has high operation difficulty.
In order to achieve the above object, the present invention provides a method for measuring a ground stress based on an optical fiber strain sensing pipe column, wherein the optical fiber strain sensing pipe column comprises:
The pipe column body is provided with an inlet end for connecting with the liquid injection pressurizing system, and can generate strain according to the internal and external pressure change; and
The optical fiber sensor is adhered to the peripheral wall of the pipe column body and can measure the strain of the pipe column body, and a plurality of sensing points which are arranged at intervals and positioning marks for marking the space positions of the plurality of sensing points are arranged on the optical fiber sensor;
The method for measuring the ground stress is applied to the optical fiber strain sensing tubular column and comprises the following steps:
Step S10: preparing an optical fiber strain sensing tubular column, and acquiring a strain correction coefficient of the optical fiber strain sensing tubular column;
Step S20: burying an optical fiber strain sensing tubular column into a rock mass;
Step S30: applying different preset pressures to the optical fiber strain sensing tubular column, and measuring actual strain values of the peripheral wall of the optical fiber strain sensing tubular column under the different preset pressures according to the strain correction coefficients;
Step S40: acquiring stratum elasticity parameters according to the actual strain value;
Step S50: and acquiring overburden formation pressure, and calculating static ground stress of the rock mass according to the overburden formation pressure and the formation elastic parameters.
In the embodiment of the invention, the outer peripheral wall of the tubular column body is provided with a threaded shallow notch, and the optical fiber sensor is positioned in the shallow notch and spirally wound on the outer peripheral side of the tubular column body along the shallow notch.
In an embodiment of the invention, the optical fiber strain sensing pipe column further comprises a film wrapped on the outer wall of the pipe column body.
In the embodiment of the present invention, step S10 includes:
Sealing the optical fiber strain sensing tubular column;
applying preset pressure into the optical fiber strain sensing tubular column through the liquid injection pressurizing system, and calculating a corrected strain actual value of the optical fiber strain sensing tubular column according to the preset pressure;
Recording the corrected strain measurement values measured at each sensing point;
And (3) performing linear correction on the corrected strain measured value and the corrected strain actual value by using an analytic method or a numerical simulation method to obtain the strain correction coefficient.
In the embodiment of the present invention, step S20 includes:
drilling a mounting hole on the rock mass;
And placing the optical fiber strain sensing pipe column in the mounting hole, and pouring cement between the inner wall surface of the mounting hole and the outer wall surface of the optical fiber strain sensing pipe column.
In the embodiment of the present invention, step S30 includes:
Injecting high-pressure liquid with different preset pressures into the optical fiber strain sensing tubular column through the liquid injection pressurizing system so as to enable the interior of the optical fiber strain sensing tubular column to generate different internal pressures;
sensing the strain of the optical fiber strain sensing tubular column by adopting an optical fiber sensor, and obtaining strain measurement values of the optical fiber strain sensing tubular column under the action of different internal pressures;
The actual strain value is obtained from the strain measurement value and the strain correction coefficient.
In the embodiment of the present invention, step S40 includes:
step S41: obtaining a strain assumption value and a formation elasticity parameter assumption value according to a functional relation between a preset pressure and a strain actual value of the optical fiber strain sensing tubular column and the formation elasticity parameter;
step S42: and repeating the step S41 until the error between the strain assumption value and the actual strain value reaches a preset error range, wherein the formation elasticity parameter assumption value is the actual formation elasticity parameter of the rock mass.
In the embodiment of the present invention, step S50 includes:
And obtaining the ratio of the static ground stress to the overburden formation pressure by using the linear relation of the formation elastic parameter and the static ground stress, and obtaining the static ground stress according to the overburden formation pressure and the ratio.
In the embodiment of the invention, the linear relation between the stratum elastic parameter and the static ground stress is as follows:
Wherein E x、Ey、Ez、υxy、υxz、υyz is stratum elasticity parameter; σ x、σy is the horizontal ground stress; σ z is the overburden pressure.
Through the technical scheme, the method for measuring the ground stress based on the optical fiber strain sensing tubular column has the following beneficial effects:
Compared with the traditional ground stress measurement, when the optical fiber strain sensing pipe column is used for measuring the ground stress, the dynamic change of the ground stress in different time periods can be continuously sensed; and the optical fiber sensor with high spatial resolution is installed in the stratum by adopting a winding type optical fiber strain sensing tubular column, and the optical fiber sensor is provided with a plurality of sensing points, and the spatial sensing resolution is as high as 1mm, so that the high spatial resolution multi-point measurement of the ground stress can be realized. In addition, after the optical fiber strain sensing tubular column is buried in the stratum rock mass, the optical fiber strain sensing tubular column can be used as a production tubular column, a structural tubular column, a fluid flow channel or a structural member in the rock mass and the like to be remained in the stratum all the time so as to realize continuous sensing of the ground stress state in different time periods without additional operation.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Drawings
The accompanying drawings are included to provide an understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate the invention and together with the description serve to explain, without limitation, the invention. In the drawings:
FIG. 1 is a flow chart of a method for measuring ground stress according to an embodiment of the invention;
FIG. 2 is a schematic diagram of a fiber optic strain sensing tubing string according to an embodiment of the present invention;
FIG. 3 is a graph showing the response of actual strain to a predetermined pressure according to an embodiment of the present invention.
Description of the reference numerals
| Reference numerals | Name of the name | Reference numerals | Name of the name |
| 10 | Optical fiber sensor | 20 | Tubular column body |
| 11 | Sensing point |
Detailed Description
Specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for purposes of illustration and explanation only and are not intended to limit the present invention.
The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to the present invention is described below with reference to the accompanying drawings.
As shown in fig. 2, in an embodiment of the present invention, an optical fiber strain sensing pipe string is provided, wherein the optical fiber strain sensing pipe string includes a pipe string body 20 and an optical fiber sensor 10, the pipe string body 20 is provided with an inlet end for connecting a liquid injection pressurizing system, and the pipe string body 20 is capable of generating strain according to internal and external pressure changes; the optical fiber sensor 10 is adhered to the outer peripheral wall of the pipe column body 20, and can measure the strain of the pipe column body 20, and the optical fiber sensor 10 is provided with a plurality of sensing points 11 which are arranged at intervals and positioning marks for marking the space positions of the plurality of sensing points 11.
When the optical fiber strain sensing pipe column is used for measuring the ground stress, the dynamic change of the ground stress in different time periods can be continuously sensed; and the optical fiber sensor 10 with high spatial resolution is installed into the stratum by adopting a winding type optical fiber strain sensing tubular column, and the optical fiber sensor 10 is provided with a plurality of sensing points 11, and the spatial sensing resolution is as high as 1mm, so that the high spatial resolution multi-point measurement of the ground stress can be realized.
When the optical fiber strain sensing pipe column is manufactured, the optical fiber sensor 10 is spirally wound on the outer peripheral wall of the pipe column body 20, and the optical fiber sensor 10 is fixed on the pipe column body 20 through an adhesive such as 502 glue or phenolic resin. At this time, the optical fiber sensor 10 is wrapped on the outer wall surface of the tubular column body 20, and a strain correspondence relationship is established between the optical fiber sensor 10 and the tubular column body 20, so that the optical fiber sensor 10 can synchronously sense the strain of the outer surface of the tubular column body 20.
When the optical fiber sensor 10 is wound, two methods of grooving positioning or fixed helix angle winding can be adopted to wind and mount the optical fiber sensor 10. The grooving positioning method comprises the following steps: the optical fiber sensor 10 is installed in the shallow notch and spirally wound on the outer peripheral side of the pipe column body 20 along the shallow notch by machining the outer peripheral wall of the pipe column body 20, so that the installation and positioning of the optical fiber sensor 10 can be realized under the condition that the strength of the pipe column body 20 is slightly influenced, the optical fiber sensor 10 can be protected by the shallow notch, the optical fiber sensor 10 is not influenced by an external interface bonding substance, and the sensing stability of the optical fiber sensor 10 is improved. The winding method for the fixed helix angle comprises the following steps: the optical fiber sensor 10 is stably fixed to the column body 20 at a predetermined position at a certain lead angle by using an optical fiber winding apparatus. The fixed helix angle method has the advantage of having no additional effect on the strength and stress distribution of the structural member of the tubular column body 20, and the notch positioning method has the advantage of better protecting the stability of optical fiber strain sensing.
Because the optical fiber sensor 10 has a plurality of sensing points 11 arranged at equal intervals due to the limitation of the physical characteristics of the optical fiber sensor 10, the optical fiber sensor 10 reflects the strain of the pipe column body 20 by measuring the spatial positions of the sensing points 11, and therefore, the initial distribution positions of the sensing points 11 on the outer peripheral wall of the pipe column body 20 are marked to determine the initial positions of the sensing points 11, so that the strain of the pipe column body 20 is obtained in the external change of the sensing points 11. The specific operation is as follows: after the optical fiber sensor 10 and the pipe column body 20 are bonded, the sensing point 11 is marked by a point pressure method or a liquid nitrogen point spraying method. The spot pressing method uses a sharp rigid object such as a probe to mark a certain spot on the optical fiber sensor 10, so that local strain is generated at the pressed spot, and uses the interval distance between the sensing points 11 to obtain the specific spatial position of the next sensing point 11, so as to deduce the spatial position of each subsequent sensing point 11; the liquid nitrogen spot spraying method is similar to the spot pressing method, and adopts a liquid nitrogen spot spraying mode to enable the optical fiber sensor 10 to locally generate obvious frequency shift so as to realize marking.
After the optical fiber sensor 10 is installed on the pipe column body 20, the pipe column body 20 needs to be wrapped by a film to shield the influence of the interface bonding substances, so that the measurement accuracy of the optical fiber sensor 10 is ensured.
In the prior art, differential strain method is widely used as a common indoor test method for the measurement of ground stress, and the theoretical basis is the memory of microcracks in the stratum: after the well hole on the rock mass is formed, the balance state of the well wall and the drilled rock mass, which is controlled by the stress of the far-field site, is destroyed, and the stress is released, so that a new stress balance state is formed. The rock elastic parameters of the well wall and the rock mass are affected by the expansion, compression, shearing and the like of a large number of microcracks contained in the rock mass under the influence of stress release, so that the ground stress state is hidden in the rock elastic parameters. The traditional differential strain method experiment object is an underground rock core which is taken to the ground, a strain gauge is firstly attached to the surface of the rock core, then hydrostatic pressure is applied to the rock mass, strain in the pressurizing process is recorded, the ratio of far-field ground stress is represented by calculating the ratio of main stress, and further far-field ground stress is calculated.
Along with the development of the sensing technology, key technical indexes such as measurement accuracy, spatial resolution and the like of the optical fiber sensing technology are greatly improved. Compared with the traditional strain measurement method, the optical fiber sensing technology has the advantages of high spatial resolution, high frequency, high accuracy and the like. Its application in the geotechnical engineering field is one of the new research hotspots. The invention combines the optical fiber strain sensing technology and the differential strain method, and invents a brand new ground stress measuring method which can be suitable for various measuring scenes such as indoor tests, underground measurement and the like.
The theory basis of the invention is the same as that of the traditional differential strain method, namely the microcracks of the rock have memory, and the ground stress state information can be stored in the elastic parameters of the rock, so that the ground stress can be inverted by utilizing the elastic mechanical parameters of the rock. The core component of the method for measuring the ground stress is the optical fiber strain sensing tubular column. When the ground stress is measured, a wellhead is firstly required to be arranged on a stratum rock body, then the optical fiber strain sensing pipe column is embedded into the stratum from the wellhead, and the optical fiber strain sensing pipe column is bonded with the stratum by using well cementation cement, so that a stress transmission relation is formed. At this time, the strain sensed by the optical fiber sensor 10 is the strain at the interface between the pipe column body 20 and the cement, and the ground stress state of the stratum is released before the pipe column is put into the stratum, so that a new equilibrium state is formed, and no strain signal is generated after the optical fiber strain sensing pipe column is put into the stratum. At this time, the high-pressure liquid is artificially injected into the pipe column body 20 by the internal pressure method, so that the optical fiber sensor 10 generates a strain signal reflecting the stratum elasticity parameter.
The strain values at the outer surface of the column body 20 at the different internal pressures can be obtained by the internal pressure method. It is noted that since the optical fiber sensor 10 has a high spatial resolution, the sensing string can obtain a huge number of strain sensing results in different time and space. At this time, inversion can be performed on the strain sensing result to obtain a variation curve of the formation elastic parameter along with the internal pressure of the tubular column body 20, so as to obtain the ground stress of a certain period of time. In addition, the invention can obtain derivative parameters such as rock anisotropic elasticity parameters.
After the ground stress of a certain period is obtained, the optical fiber strain sensing tubular column is buried in the stratum rock mass and can be used as a production tubular column, a structural tubular column, a fluid flow channel or a structural member in the rock mass and the like to remain in the stratum. When the earth stress in the stratum changes with time, the outer surface of the optical fiber strain sensing tubular column generates strain and sends out corresponding strain signals. The strain signal is continuously monitored, and the ground stress change can be inverted, so that dynamic ground stress change results on different time axes are obtained, and the ground stress state is continuously sensed in different time periods.
It should be noted that, in order to form the high internal pressure of the pipe string body 20, it is necessary to ensure that the pipe string body 20 is of a sealing structure, so that physical seals are required to be formed at two ends of the pipe string body 20, and the pipe string body 20 can be sealed by selecting a packer seal or end welding mode according to different ground stress measuring environments.
As shown in fig. 1, in order to further understand the technical solution of the present application, the following describes in detail the overall working steps of the method for measuring the ground stress, which includes:
Step S10: preparing an optical fiber strain sensing tubular column, and acquiring a strain correction coefficient of the optical fiber strain sensing tubular column;
Step S20: burying an optical fiber strain sensing tubular column into a rock mass;
Step S30: applying different preset pressures to the optical fiber strain sensing tubular column, and measuring actual strain values of the outer peripheral wall of the optical fiber strain sensing tubular column under the different preset pressures according to the strain correction coefficient, wherein the range of the preset pressures is about 0-25MPa;
Step S40: acquiring stratum elasticity parameters according to the actual strain value;
Step S50: and acquiring overburden formation pressure, and calculating static ground stress of the rock mass according to the overburden formation pressure and the formation elastic parameters.
Compared with the traditional ground stress measuring method, the method adopts the optical fiber strain sensing tubular column as a sensing medium to sense the strain of the stratum rock mass, so as to reflect the stratum elastic parameters and the ground stress. The ground stress measuring method can continuously sense the dynamic change of the ground stress in different time periods; the optical fiber sensor 10 with high spatial resolution is installed in the stratum by adopting a winding type optical fiber strain sensing tubular column, and the optical fiber sensor 10 is provided with a plurality of sensing points 11, and the spatial sensing resolution is as high as 1mm, so that the high spatial resolution multi-point measurement of the ground stress can be realized; in addition, the method for measuring the ground stress has low operation difficulty, and the optical fiber strain sensing pipe column can be used as a production pipe column, a structural pipe column, a fluid flow channel or a structural part in the rock body and the like to be remained in the stratum all the time after being buried in the stratum rock body, when the dynamic numerical value of the ground stress under different time periods is required to be measured, the stratum elastic parameter is inverted for a new round only through the newly measured actual value of the strain, so that dynamic ground stress change results on different time axes can be obtained, and the continuous sensing of the ground stress state in different time periods is realized. The method effectively solves the technical problem that the ground stress measurement method in the prior art can only carry out static measurement on the ground stress in a single time period, and does not need additional operation.
In an embodiment of the present invention, step S10 includes preparing an optical fiber strain sensing string and obtaining a strain correction coefficient of the optical fiber strain sensing string. Since the fiber body is strained to a rayleigh back-scattered light frequency shift value, the measured value obtained by the fiber sensor 10 is different from the actually generated strain value and has a linear relationship therebetween, so that the measured value needs to be corrected to the actual strain value of the outer surface of the pipe string body 20 by using a strain correction coefficient.
In this case, the strain correction coefficient needs to be obtained by an internal pressure method. Specifically, a preset pressure is applied to the optical fiber strain sensing tubular column through a liquid injection pressurizing system, an analytic solution or a numerical simulation method is utilized to calculate the space strain of the internal pressure tubular column according to the preset pressure, a corrected strain actual value of the optical fiber strain sensing tubular column is obtained, then the corrected strain measured value measured by each sensing point 11 is recorded, and finally the corrected strain measured value and the corrected strain actual value are subjected to linear correction through an analytic method or a numerical simulation method, so that the strain correction coefficient is obtained.
After the strain correction coefficient is obtained, the strain measurement value detected by the optical fiber sensor 10 can be changed into a strain actual value, so that the detection precision of the strain of the optical fiber sensor 10 to the tubular column body 20 is greatly increased, and the measurement effect of the ground stress measurement method is better and more accurate.
In the embodiment of the present invention, step S20 includes:
And drilling a mounting hole on the rock body, placing the optical fiber strain sensing tubular column in the mounting hole, and pouring cement between the inner wall surface of the mounting hole and the outer wall surface of the optical fiber strain sensing tubular column. The optical fiber strain sensing pipe column and the stratum are bonded through cement, so that seamless connection is realized between the optical fiber strain sensing pipe column and the stratum, and further a stress-strain transfer relation is formed between the optical fiber strain sensing pipe column and the stratum, so that the optical fiber strain sensing pipe column can be synchronous with the strain of the stratum, and the strain measured by the optical fiber sensor 10 can better reflect the ground stress state in the stratum.
In the embodiment of the present invention, step S30 includes:
Injecting high-pressure liquid with different preset pressures into the optical fiber strain sensing tubular column through the liquid injection pressurizing system so as to enable the interior of the optical fiber strain sensing tubular column to generate different internal pressures; sensing the strain of the optical fiber strain sensing tubular column by adopting an optical fiber sensor 10, and obtaining strain measurement values of the optical fiber strain sensing tubular column under the action of different internal pressures; and finally, obtaining the actual strain value of the optical fiber strain sensing tubular column according to the strain measurement value and the strain correction coefficient obtained in the previous step.
At this time, after a preset pressure is applied in the optical fiber strain sensing pipe column, the combination of the pipe column body 20, the concrete bonding layer and the stratum is deformed at the same time, and the stratum is strained under the action of the preset pressure of the optical fiber strain sensing pipe column, so as to obtain a response curve of the actual strain value to the preset pressure as shown in fig. 3.
The sensing points 11 on the optical fiber strain sensing pipe column can measure a plurality of actual strain values, and are concentrated in fig. 3, so that the sensing points 11 at a plurality of different spatial positions are detected, the relation between the actual strain values and the preset pressure can be more accurately reflected, and the accuracy of a measurement experiment is improved.
In the embodiment of the present invention, step S40 is an inversion process of the formation elastic parameter, including:
Step S41: according to the relation between the preset pressure and the actual value of the strain of the optical fiber strain sensing tubular column and the stratum elasticity parameter, the strain assumption value and the stratum elasticity parameter assumption value can be obtained by utilizing a finite element method;
Step S42 is: and continuously repeating the step S41, and obtaining new assumed strain values and assumed formation elastic parameter values until the error between the assumed strain values and the actual strain values reaches the tolerance range, so that the actual formation elastic parameter of the rock mass is the same as the assumed formation elastic parameter value.
The inversion process of the stratum elastic parameters can be performed by using two modes of a numerical method and an analysis method aiming at the inversion process of the stratum elastic parameters under different preset pressures. The numerical method is to invert stratum elasticity parameters by using numerical simulation software or algorithm; the analytic method is a method for inverting the elastic mechanical parameters by utilizing an analytic solution model. Since the theoretical basis of the numerical method and the analytical method is established in the above step S40, the numerical method and the analytical method will not be described in detail herein.
In addition, the inversion process of the stratum elastic parameters can be arithmetically optimized by using an optimization method, a machine learning method and a deep learning method. The optimization method comprises particle swarm method, simulated annealing method, snake optimization, whale optimization and other evolutionary algorithms. Common methods for machine learning include random forests, support vector machines, and the like. The deep learning method comprises BP neural network, neural network combined with optimization algorithm and the like. The inversion process of elastomehc may be optimized in various ways within the scope of the technical idea of the present invention, including the specific optimization methods described above being combined in any suitable way, or other conventional optimization algorithms, and the present invention will not be described in any way. But such algorithms should be considered as the disclosure of the present invention as well.
It should be noted that before the wellhead is opened in the stratum to embed the optical fiber strain sensing tubular column, the stratum is extruded by the stress of the periphery of the well bore, so that the stratum fracture is in an excessively closed state. After the optical fiber strain sensing tubular column is embedded into the stratum from the well bore, in the process of internal pressurizing expansion, stratum cracks around the well bore are stressed to be reopened, so that the density of the stratum in the pressurizing process is continuously reduced, and the gradient of the curve is continuously increased in FIG. 3.
Specifically, the curve of fig. 3 can be sequentially divided into a first linear region, a nonlinear region and a second linear region along with the increase of the preset pressure, when the preset pressure is located in the first linear region, the stratum fracture is in an excessively closed state, the stratum elastic parameter is a constant value, the slope of the curve is unchanged, and the stratum is in an isotropic state; when the preset pressure is in a nonlinear region, the stratum fracture is in a gradually opened state, stratum elasticity parameters are in nonlinear change, and the slope of the curve is gradually increased; when the preset pressure is positioned in the second linear region, the stratum fracture is in a complete open state, the stratum elastic parameter is a constant value, the slope of the curve is unchanged, and the stratum is in an orthotropic state.
Inversion of the formation elastic parameters is performed on the data of the first linear region by using the step S40, so as to obtain the formation elastic parameters: E. v, wherein E is the isotropic formation elastic modulus; and v is the poisson ratio of the isotropic stratum.
Inversion of the formation elastic parameters is performed on the data of the second linear region by using the step S40, so as to obtain the formation elastic parameters: e x、Ey、Ez、υxy、υxz、υyz, wherein E x、Ey、Ez is the orthotropic formation elastic modulus; v xy、υxz、υyz is the orthogonal anisotropy formation poisson's ratio.
In the embodiment of the present invention, step S50 includes:
and combining the overburden formation pressure, obtaining the ratio between the static ground stress and the overburden formation pressure by using a linear relation of the formation elastic parameter and the ground stress, and obtaining the static ground stress of the formation according to the overburden formation pressure and the obtained ratio.
Wherein, the overburden formation pressure can be calculated by the following formula:
σz＝Hρrg
Wherein sigma z is the overburden formation pressure, and H is the depth of the sensing point 11 in the formation; ρ r is the total density of the formation, ρ r can be measured by density logging; g is gravitational acceleration.
The linear relation between the stratum elasticity parameter and the ground stress is:
Wherein σ x、σy is the horizontal ground stress. According to the above equation and the formation elasticity parameter determined in step S40: e x、Ey、Ez、υxy、υxz、υyz, finally obtaining the ratio of sigma x to sigma z、σy to sigma z respectively, further obtaining the specific values of horizontal ground stress sigma x and sigma y, and completing the measurement of the ground stress (including horizontal ground stress and overburden stratum pressure).
In general, the invention can calculate different ground stress of the positions of different sensing points 11 according to the strain measured by different sensing points 11 on the optical fiber strain sensing tubular column, thereby realizing multi-point measurement of the ground stress, avoiding complex operation of the traditional measurement method and greatly reducing the measurement difficulty of the ground stress.
It should be noted that the ground stress and the elastic parameter of the stratum are not constant values, but continuously change along with time, and the ground stress measuring method of the invention not only can measure static ground stress in a certain period, but also can be used as a production string, a structural string, a fluid flow channel or a structural member in a rock mass, etc. to stay in the stratum all the time. When the ground stress and the stratum elastic parameters in the stratum change along with time, the outer surface of the optical fiber strain sensing tubular column generates strain and continuously measures new actual strain values, and the stratum elastic parameters and the ground stress can be inverted again to obtain the stratum elastic parameters on different time axes, and continuous sensing of the ground stress state in different time periods is realized. The method effectively solves the technical problem that the ground stress measurement method in the prior art can only carry out static measurement on the ground stress in a single time period.
In the description of the present invention, it should be understood that the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise.
In the present invention, unless explicitly specified and limited otherwise, the terms "mounted," "connected," "secured," and the like are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally formed; may be mechanically connected, may be electrically connected or may be in communication with each other; either directly or indirectly, through intermediaries, or both, may be in communication with each other or in interaction with each other, unless expressly defined otherwise. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art according to the specific circumstances.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
While embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and not to be construed as limiting the invention, and that variations, modifications, alternatives and variations may be made to the above embodiments by one of ordinary skill in the art within the scope of the invention.
Claims (7)
1. The method for measuring the ground stress based on the optical fiber strain sensing tubular column is characterized by comprising the following steps of:
The pipe column body (20) is provided with an inlet end used for being connected with a liquid injection pressurizing system, and the pipe column body (20) can generate strain according to internal and external pressure changes; and
The optical fiber sensor (10) is adhered to the peripheral wall of the tubular column body (20) and can measure the strain of the tubular column body (20), and a plurality of sensing points (11) which are arranged at intervals and positioning marks for marking the space positions of the plurality of sensing points (11) are arranged on the optical fiber sensor (10);
the method for measuring the ground stress is applied to the optical fiber strain sensing tubular column, and comprises the following steps:
Step S10: preparing an optical fiber strain sensing tubular column, and acquiring a strain correction coefficient of the optical fiber strain sensing tubular column;
step S20: burying the optical fiber strain sensing tubular column into a rock mass;
step S30: applying different preset pressures to the optical fiber strain sensing tubular column, and measuring actual strain values of the peripheral wall of the optical fiber strain sensing tubular column under the different preset pressures according to the strain correction coefficient;
Step S40: acquiring stratum elasticity parameters according to the actual strain value;
Step S50: obtaining a ratio between the overburden formation pressure and the static ground stress by utilizing a linear relation between the static ground stress and the formation elastic parameter, obtaining the overburden formation pressure, and obtaining the static ground stress according to the overburden formation pressure and the ratio;
wherein the linear relationship is:
Wherein E x、Ey、Ez、υxy、υxz、υyz is stratum elasticity parameter; σ x、σy is the horizontal ground stress; σ z is the overburden pressure.
2. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the outer peripheral wall of the tubular column body (20) is provided with a threaded shallow notch, and the optical fiber sensor (10) is positioned in the shallow notch and spirally wound on the outer peripheral side of the tubular column body (20) along the shallow notch.
3. The method of claim 2, further comprising wrapping a film around an outer wall of the column body (20).
4. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S10 comprises:
sealing the fiber optic strain sensing tubing string;
applying preset pressure into the optical fiber strain sensing tubular column through a liquid injection pressurizing system, and calculating a corrected strain actual value of the optical fiber strain sensing tubular column according to the preset pressure;
recording the corrected strain measurement values measured at each of the sensing points (11);
and linearly correcting the corrected strain measured value and the corrected strain actual value by using an analytic method or a numerical simulation method to obtain a strain correction coefficient.
5. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S20 comprises:
drilling a mounting hole on the rock mass;
And placing the optical fiber strain sensing pipe column in the mounting hole, and pouring cement between the inner wall surface of the mounting hole and the outer wall surface of the optical fiber strain sensing pipe column.
6. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S30 comprises:
Injecting high-pressure liquid with different preset pressures into the optical fiber strain sensing tubular column through a liquid injection pressurizing system so as to enable the interior of the optical fiber strain sensing tubular column to generate different internal pressures;
Sensing the strain of the optical fiber strain sensing tubular column by adopting an optical fiber sensor (10), and obtaining strain measurement values of the optical fiber strain sensing tubular column under the action of different internal pressures;
And obtaining the actual strain value according to the strain measurement value and the strain correction coefficient.
7. The method for measuring the ground stress based on the optical fiber strain sensing tubular column according to claim 1, wherein the step S40 comprises:
Step S41: obtaining a strain assumption value and a formation elasticity parameter assumption value according to a functional relation between the preset pressure and the actual strain value of the optical fiber strain sensing tubular column and the formation elasticity parameter;
Step S42: and repeating the step S41 until the error between the strain assumption value and the actual strain value reaches a preset error range, wherein the formation elasticity parameter assumption value is the actual formation elasticity parameter of the rock mass.
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